Communicating designs with your customers and suppliers is vital to ensure demands are met during the development stage. In the past you would have had to ensure you were working with the same SOLIDWORKS version to share data with ease. In SOLIDWORKS 2013 files created and saved can be opened in the last service pack (SP5) of 2012. This is called "Interoperability". It will allow you to view "newer" 2013 parts and assemblies and their configurations, to allow you to reference them in your 2012 file sets- i.e. add mates to them, and create geometry from them.
Please note drawings cannot be opened in the previous versions, and feature history (i.e. sketches) will not be available, but it does mean the data can be shared in the native format back and forth- with updates and changes reflected. This trend will also continue going forward- 2013 SP5 will be able to open 2014 files.